An Iranian Asmari reservoir with an initial oil-in-place of about 381 millions m3 (2.4 billion bbls) and a history of18 years of oil production is studied in this work. The field is a typical Iranian fractured carbonate oil reservoir which has a matrix rock of poor quality and a highly permeable fracture network.
The subject of the study was to simulate the reservoir flow performance with an elaborated two-dimensional vertical model capable of simulating nonconventional producing mechanisms such as gravity drainage and block to block interaction. After successful proving of the model capability by reservoir production history the model was used to predict the reservoir behaviour under natural depletion, and to evaluate two ases of pressure maintenance by gas injection.
Results showed that in both cases of pressure maintenance, an ultimate additional oil production of about 150 million bbls will be achieved over natural depletion. This additional oil production corresponds to an increase of approximately 6 % of original oil in place.
The oil reservoir under study is an elongated NW-SE anticline symmetrical along its minor axis located in southwest of Iran. The structure plunge to the southeast is more gentle than to the northwest. The productive area is 195 kilometres long and 7.75 kilometres wide at the original WOC located at 1890 meters subsea. Heavy mud losses during drilling, production tests, and good pressure communication between the wells indicate the existence of an active fracture system. The original reservoir pressure at a depth of 1600 loss, was measured to be 210.7 bars about 57 bars above saturation pressure.
Commercial oil production commenced in 1973 at a rate of 3100 m3/day. The rate was then successively increased as more wells were drilled in the field. During the early time of the field production, a relatively fast pressure drop as shown in Fig. 1 was encountered which is characteristic of an undersaturated reservoir. In 1978 the pressure at the crest had been dropped below the bubble point pressure ausing the liberation of solution gas and the formation of a secondary gas cap.
The field was closed-in from 1979 to 1984 during which 0.2 billion m3 of gas had been injected intermittently. During this period the average reservoir pressure was increased by 20 bars approximately. Then the production started at a rate of 6000–6500 m3/D with a gas injection rate of 1.4 million m3/D continuously. From 1986 the amount of injected gas was insufficient to maintain the reservoir pressure, therefore the reservoir pressure was lightly declined. Fig. 2 depicts the trend of pressure and the reservoir production history.
The producing mechanisms in the fractured reservoirs of Iranian type are substantially different from those in conventional homogeneous reservoirs. Therefore to study their performance special mathematical model should be used.
In this study It will be shown that the two-dimensional fractured reservoir model "STACKED BLOCK MODEL" which describes the reservoir as a series of stack of interacting blocks, surrounded by fracture space, can be successfully applied to simulate the performance of the reservoir.